Breaking Eggs

In order to do good research, sometimes you have to break some eggs. In this case, in order to write a good controller for the PR2 gripper, Penn's Matt Piccoli has been breaking a lot of eggs. Matt is on his second stint at Willow Garage as an intern, and, together with Sachin Chitta, he crushed a lot of eggs with the PR2 gripper to figure out just how much force it takes to crack an egg. They were able to use this data to develop a better grasp controller for the PR2 gripper. They wrote a closed loop controller that switches between two modes: it starts as a velocity controller, but it then switches to a force controller when it senses the egg in its grasp using its fingertip sensors. This new controller has many uses beyond grasping eggs. By starting with a more difficult object like an egg, they can now use the same controller for less delicate objects as well.


linear elastic actuation

I'll preface by indicating I do not know the current technology used in the hand (and therefore may be mentioning something that is already implemented). That said, if you are trying to accomplish force control, series elastic actuation is certainly something to look at. Put a spring in series with the actuator and you're able to control force through a position control loop.

Scrambled eggs

Have you tried impedance control (Hogan)? It is good for such a problem where contact and switching control modes can lead to instability.

Speed of velocity controller

The velocity controller is noticeably slow because you can't anticipate when you'll reach the egg, so you have to keep your momentum low in order to ensure that the force on the egg is small. However, if you modify your gripper by making it springier, much as a system of tendons and muscles is springy, then the force will ramp up more slowly when the gripper reaches the egg, giving your controller more time to react by slowing the gripper. That way, you can close the gripper faster while still maintaining a safety margin. Sometimes having the right hardware simplifies control.